Single unit responses in cochlear nucleus of horseshoe bats to sinusoidal frequency and amplitude modulated signals

Abstract

1. In the cochlear nucleus (CN) of Horseshoe bats, recordings were made from 148 single neurons to constant frequency (CF) signals and to sinusoidally frequency modulated (SFM) and sinusoidally amplitude modulated (SAM) signals, which in a first approximation simulate the periodic modulations of echoes returning from wing beating insects. 2. The probability of obtaining synchronized discharge activity to modulations was independent of the neurons' best frequencies (BFs). Neurons belonging to separate response pattern categories to CF-signals differed in the synchronization pattern to periodic modulations. While the response patterns of tonic neurons were characterized by an image-like reproduction of the time course of frequency or amplitude changes, phasic neurons respond with an activity peak to distinct portions of the modulation cycles. Neurons with build up patterns to CF-signals (n= 8) and three phasic neurons did not lock their discharges to SFM- or SAM-signals. Response patterns were essentially similar for SFM- and SAM-signals and systematically depended on modulation magnitude, carrier frequency and intensity parameters of the signals. 3. The effects of decreasing modulation magnitude were tested for 45 neurons with SFM- and for 46 neurons with SAM-signals. Asymmetries in the response patterns present at high modulation magnitudes disappeared with smaller modulation magnitudes and histogram envelopes approach sinusoidal form. Synchronized responses to SFM-signals were present down to modulation heights of ±20 Hz and to SAM-signals of 6%. Sensitivity for small modulation heights of the SFM-signal was highest in sharply tuned neurons in the frequency range of 81–88 kHz (‘filter neurons’). No correlations between minimum modulation depth of the SAM-signal and the neurons' BFs were found. 4. The carrier frequency of the SFM-signal was systematically varied in 19 neurons. Sensitivity for small modulation heights could be improved by positioning the carrier frequency near the flanks of the excitatory response area. The response to SAM-signals was less influenced by the carrier frequency. 5. The intensity parameter was studied in 18 neurons with SFM- and 25 neurons with SAM-signals. While synchronization was clearly present in the range of 10 to 30 dB SPL above minimum threshold, the response peaks to individual modulation cycles of tonic neurons tended to fuse at higher intensities. In sharply tuned neurons, individual response peaks to SFM-signals could be distinguished over wider intensity ranges than with SAM-signals. 6. Periodically amplitude modulated SFM-signals with different phase relations between amplitude maximum and particular frequency components were used for stimulation of 22 neurons. Due to their sharp tuning properties, the response pattern of filter neurons systematically changed depending on the phase relations of AM and FM, whereas the response pattern of neurons in other frequency ranges resembled the pattern to SAM-stimuli. 7. In order to determine the limiting rate of frequency and amplitude change for synchronization of response activity, the modulation frequency was varied between 50 Hz and 2,000 Hz in 59 neurons with SFM- and 46 neurons with SAM-stimuli. The range of modulation frequencies covered by synchronized activity was broad, with most neurons (n=41 SFM,n=28 SAM) reaching upper rates between 400 and 800 Hz, or even higher (n=6 SFM,n=14 SAM). Neurons with limiting rates below 400 Hz were rare (n=12 SFM,n=4 SAM). Low, medium and high limiting rates were found in tonic as well as phasic neurons. Some phasic neurons (n=3), required minimum modulation frequencies of 200 Hz to start synchronization. Synchronization behavior depended on intensity, center frequency, modulation magnitude and could be different for SFM- and SAM-signals in the same unit. 8. Data are discussed in relation to single unit recordings to periodically modulated signals in CN of other mammals and to results obtained from higher order auditory nuclei of Horseshoe bats.